Crystal unit inductance adjustment



Sept. 18, 1956 D. F. CICCOLELLA CRYSTAL UNIT INDUCTANCE ADJUSTMENT FiledJan. 6. 1950 w/o TH 0F s ur m 7, OF CRYSTAL BLANK WIDTH M) VEN 70/? D. EC/CCOLELLA A T TORNEV nited States CRYSTAL UNIT INDUCTANCE ADJUSTMENTApplication January 6, 1950, S'eri'alNo. 137,243

1 Claim. (Cl. 29-4535);

This invention relates to the production of piezoelectric crystalshaving divided coatings and, more particularly, to a method andapparatus for improving the inductance adjustment of such crystals.

It is known that adjustment of the impedance of coated piezoelectriccrystals can be made. by separating portions of the coatings from eachother and by connecting, the divided sections in certain preferredmanners. The factors involved in such impedance adjustments are.described in Patent 2,194,539, issued March 26, 1940, to J- F. Barry andH. G. Och, which. explains how a variety of circuit arrangements andcrystalconstants can be. obtained by appropriate divisions of thecoating and interconnections of the separate sections.

The means by which such separations between various coating areas wereobtained in the prior art included methods of removing the coating, suchas grinding with a sharp-bladed wheel or chemical etching, which werelikely to change the characteristics of the crystalline material alongthe division path. For example, chemical etching frequently removed aportion of the surface of the crystal as well as the plating, and thuschangedthe frequency or introduced spurious vibration. Similarly,grinding may disturb the crystalline structure adjacent the groundportion and thus change the vibrational characteristics of the crystalblank.

The present invention pertains to means and methods for adjusting theinductance and dividing; the coating which avoids interference with thesurface structureof the crystal blank and reduces the number of stepsrequired improduction. It contemplates removing: a desired width. of thecoating along the intended line of division: by means of a fine air jetloaded with a mild abrasive and directed against the line of divisionthrough a carefully dimensioned mask.

Apart from the method used. to remove; the coating along the intendeddivision line, the prior art has involved two steps in the adjustment ofinductance in. splitplated crystals. The inductance adjustment hascommonly been made commercially only in substantially rectangularcrystals. In such crystals, usually the coating is divided along eitherthe longitudinal or transverse axis of a major face or faces. Theinitial step has been that of making a transverse split near one end to"adjust the inductance, followed by a second step comprising thelongitudinal or transverse dividing of the main coatings. Each of theseoperations has had to' be carried on with the use of a separate set ofjigs and fixtures, and involved handling the crystal a number of timesduring theproduction. The inductance adjustment had to account for thechange in inductance value which would subsequently be produced by thesecond step, that is, the longitudinal or transverse division of themain coatings.

The fact that two operations, both of which were held to the sametolerances in accuracy, were required raised the cost of production andled to a doubled possibility of rejection. The method hereafterdescribed reduces atent 2. thisv possibility, and the cost, by requiringonly asingle operation to accomplish both the splitting and inductanceadjustment.

The present invention contemplates the attainment in one operation ofthe inductance adjustment and the longitudinal or transverse division ofthe coatings through careful predetermination of the adjustment in thewidth of the longitudinal or transverse division. This eliminatesseveral steps in handling and several sources of inaccuracy, inasmuch asthe entire adjustment tov the proper value of inductance is obtainedduring the single step, and it is unnecessary to allow separately, invadjusting inductance, for the subsequent dividing operation, which mightagain change the inductance value of the crystal.

The method of providing the entire adjustment for inductance bycontrolling the. width of the longitudinal or transverse division in thecoating is facilitated by the use of a fixture comprising a stand onwhich the crystal may be securely held, and a masking plate having thewidth of the aperture therein accurately predetermined.

When it is desired to adjust the inductance of a coated crystal blank,the proper width of coating to be removed in order to obtain the desiredinductance is determined. From a group of masking plates having sharplydefined apertures formed therein, in a series covering all necessaryremoval area widths, the proper masking plate is selected and placedover the crystal on its stand. A finely pointed air brush arranged to befed by a mixture of air and a mild abrasive, such as finely powderedpumice, stone or pounce, is directed against the coating through themask. It has been found that by this method, it is possible to removethe desired width of line accurately and quickly without in any waydamaging or changing the character of the surface of the quartzimmediately therebeneath.

The determination of the width of the division necessary to accomplishthe desired change in the value of inductance has been determinedempirically, and graphs made for the guidance of an operator which showthe inductance step-up which will be obtained by removing any particularwidth of coating.

A primary object of the invention is to improve the technique ofmanufacturing coated crystals by combining in one operation theadjustment of crystal inductance, and the dividing of the coating.

A further object is the reduction of the cost of manufacture of crystalshaving divided coatings.

Another object is reduction of possible errors in acl justment byreducing the number of steps in which separateerrors may appear.

Yet another object is the elimination of the necessity for applying, inone step, a correctional factor to take account of alater step-inproduction.

A still further object is toprovide a simple and inexpem sive fixturefor use in carrying out the dividing operation.

An additionalobject is to provide such a fixture in a form which willpermit dividing a coating either longitudinally or transversely.

Another object is the provision of a form of fixture which will allowthe division process to be accomplished on crystals to which connectingwires have already been attached, without disturbing the electricalcontinuitybetween the coating and the wires.

A further object is the improving of the technique of dividing thecoating on. piezoelectric crystals Still another object is the provisionof a method of dividing crystal coatings without deleteriously affectingthe surface of the crystal.

An additional object is the provision of a method of inductanceadjustment which is applicable to machinery in which the width of themasking aperture may be ice adjusted as needed over a continuous range,by an operator.

These and other objects are achieved by means of the embodimentsillustrated and described hereafter. .lt-is to be understood that theseembodiments are exemplary only, and it is contemplated that other formsand shapes of fixtures, other materials, and variations in the method ofproducing the divided coatings are included within the scope of theinvention.

The invention may be better understood from the following detaileddescription with reference to the drawings in which:

Fig. l is an exploded perspective view of a stand for holding thecrystal, a crystal blank and a masking plate, separated from theirnormal assembled position for clarity in illustrating the relation ofthe parts;

Fig. 2 is a fragmentary sectional view taken along line 2-2 of Fig. l toshow the masking plate in cross section;

Fig. 3 is a top elevation of a coated crystal in which the inductancehas been adjusted and a longitudinal division in the coating made by theuse of the method and apparatus hereinafter described.

Fig. 4 is a perspective view of a fixture for use in practicing theinvention; and

Fig. 5 is a graphical representation showing the change in inductance asa function of the width of the division in terms of percentage of thewidth of the crystal blank.

Referring now to Fig. l of the drawings, there is disclosed a stand 1comprising a bed plate 2 and supporting legs 4. Bed plate 2 islongitudinally and transversely grooved at 5 and 6 respectively toreceive a coated crystal 7 in either of two alternative positions.Alignment pins 9 extend upwardly from the surface of plate 2. A coverplate 10 is aligned properly on the stand 1 by bores 11, which cooperatewith and fit over pins 9. Plate 10 fits snugly over the crystal 7, andserves by virtue of its weight to retain the crystal in place during thedividing operation. The cover or mask plate 10 is provided with abeveled slot 12 carefully machined to have exactly the width of openingdesired. It is to be understood that a complete fixture set includes anumber of plates 10, each having a slot 12 of different width and theslot widths being graduated to cover the necessary range by smallincrements. Slot 12 is cut parallel to the longitudinal axis of plate10, so that when a crystal is held in recess 5, a longitudinal divisionof the coat ing may be obtained, and when the crystal is set in recess6, a transverse division may be produced.

The masking plate 10, as shown in detailed section in Fig. 2, has, inaddition to the slot 12, a number of bores 13, extending along thetransverse midline of the plate. The bores 13 are recessed orcountersunk at 14 on the underside of the mask to permit use with blanksto which connecting and supporting leads 15 have already been attached.These recessed portions 14, which may be conical as shown in Fig. 2,provide clearance between the mask and the areas of attachment of theconnecting wires. Where such means for connecting the crystal in thecircuit are to be used, the inductance adjustment is normally made afterthe connecting wires have been attached to the crystal plate.

The techniques commonly used for the attachment of ered temporarily bystrips of a pressure-sensitive or ad hesive tape 16. Clearance for theconnecting wires ex tending from the lower side of the crystal as itrests on bed plate 2 may be obtained by providing large clearance bore17 centrally of the bed plate.

A considerable variation in crystal length is possible with the standand cover shown in Fig. l, as indicated by the dotted outline 8. Themaximum length of a particular division line is of course the length ofthe slot 12. lt'will be obvious, however, that with varying widths andthicknesses of crystals, additional means must be used to obtain properalignment and a snug fit between the crystal andthe masking plate. ,7

An adapter'20 suitable for this purpose is shown in Fig. 4, and it iscontemplated that it-will be used with a stand similar to the type shownin Fig. 1, but in which the bed plate 2 is substantially thicker, andthe recesses 5 and 6 correspondingly deeper and wider than in thatshowing. I

There will befrequired a set of adapters, 20, one for each width andthickness of crystal to be processed. All the adapters in the serieswill have identical outer dimensions in width 21 and thickness 22, sothey may fit interchangeably in the stand recesses 5 and 6. Each willhave the wires involve a contact area greater than the diameter of thewire. Whether this increased area is obtained by upsetting the wire end,using a relatively large amount of solder or cement, or by other means,it is desirable to protect the bond. Different connecting wire spacingsare provided for by making a series of clearance bores a central bore 26for the accommodation of connecting leads.

Each, however, will have a difierent combination of seat width 24 andseat depth 25, tailored to fit the anticipated dimensions of thedifferent crystals to be processed, so that the mask will fit snuglyagainst the crystal surface, and the crystal will be accurately alignedwith the intended division line.

The actual removal of the coating along the intended line of division isthen accomplished by directing a fine air jet, loaded with a pumice orother mild abrasive, against the mask along the aperture 12.

An air gun of the type known commercially as the Paasch air brush issatisfactory for this use, although any other means of providing aconcentrated air jet may be used.

A crystal blank 7 is shown in Fig. 3 after the coating has been removedalong the intended division line 27.

By eliminating the separate inductance and splitting operation, a savingis achieved in manufacture which amounts to about 50 per cent of thepresent cost of splitting and adjusting the inductance, and the use ofprepared graphs to select the width of plating to be removed simplifiesand expedites the operation, especially since this eliminates thenecessity for providing in the adjustment of inductance step for thechange in inductance produced by the longitudinal splitting operation.In prior methods, this was of necessity an approximation, since thewidth of the inductance adjustment determined the length of thelongitudinal plating division, and hence in turn introduced a possiblesecond order error in the final provision for inductance adjustment.

In processing two groups of Y-cut EDT crystals by the method hereindisclosed, it was found that in one group which required an inductancestep-up of 9 per cent, the final inductance measurements indicated thatall units in'the group were within -1.5 per cent of the desired normalinductance. The second group which required an inductance step-up of 20per cent gave a final measurement within :L-Z per cent of the desirednormal inductance for all of the units in the group.

It will be apparent that the method described of adjusting theinductance by a single step could be applied to machinery in which thewidth of the mask slot could be easily adjusted by an operator toprovide the desired change in inductance so that the method is welladapted to mass-production technique.

What is claimed is:

The method of simultaneously dividing the metallic electrode coating andadjusting the inductance of an e ect o s tsd P zoel ct sr alw p s h d nall of the metallic coated area of a major face of a piezoelectriccrystal except for a strip across the full length of the metalliccoating, said strip of a Width directly proportional to the inductancestep up desired, and directing a stream of compressed air and mildabrasive toward the exposed portion of the metallic coating to removesaid exposed portion Without interference with the underlying crystalstructure.

References Cited in the file of this patent UNITED STATES PATENTS 6Fritsche Dec. 19, 1933 Beard Oct. 22, 1935 Barry Mar. 26, 1940 PrangeOct. 20, 1942 Lindenmeyer Aug. 31, 1943 Wolfskill Apr. 4, 1944 FruthOct. 16, 1945 Williams June 10, 1947 Thompson Sept. 28, 1948 FOREIGNPATENTS Australia Feb. 21, 1946

